Installation of 4-synchro gearbox into the Mk1 MGB body

 In MGA-MGB, Midget, Transmission And Rear Axle

Note: In addition to the changes below, cars with 3 bearing engines have a much smaller crankshaft spigot bush (5/8″ against 7/8″). As far as I’m aware, the only way around this is to reduce the diameter of the first motion shaft to match the early one. This however raises questions over the hardness of the resulting bearing surface on the modified shaft, and as I have not carried out this particular operation I’ve never been in a position to investigate it. If contemplating such a change, it is essential that this is checked out!  It may be possible to enlarge the crankshaft spigot if the engine is out and stripped, but care would have to be taken that the oilway from the rear main to no 4 bigend.


Overdrives are a polular retro-fit option for those MGB’s which were not fitted with one from new, particularly as cruising speeds now are higher thanks to modern roads being much faster than those in the period.

The early D-type overdrive fitted to the 3 synchromesh gearbox of the Mk1 MGB is a relatively rare beast, so a very viable alternative is to fit the later full synchromesh gearbox and overdrive.

By and large it is a relatively direct swap – even the overall length of the later gearbox is the same as the 3 synchro non-overdrive unit, so the propshaft doesn’t need changing if you presently have the non-overdrive version.  The only significant differences between the two models are:

  • The gear lever position is a bit further back
  • The flywheel, engine backplate and starter are different
  • For cars presently fitted with a 3 synchro overdrive gearbox, the propshaft needs to be shortened by an inch (or swapped for another propshaft – see below).


For cars fitted with the 5 main bearing engine, there are two ways of approaching the differences between flywheels & backplates – either retain the original ones on the engine, or to change to the later flywheel & backplate.  3 main bearing cars have no option but to retain the originals as the flywheel mounts are different.

A relevant consideration is that the bellhousing of the full synchromesh gearbox was wider and enabled a larger flywheel and pre-engage starter to be fitted.   However, the transmission tunnel was also wider, and the complete assembly of gearbox, flywheel & starter is a tight fit in the Mk 1 bodyshell.

Because of this, together with the cost and/or availability of getting the different flywheel, backplate & starter, adapting the “new” gearbox to the existing backplate is a good alternative.  Fortunately, most of the bellhousing bolts are in the same positions in both 3 and 4 synchro gearboxes, so it is realatively simple to do.  The changes needed are detailed further down the page.

Gear lever position

The gear lever being about 4″ further back than the 3 synchro non-overdrive box, so this will need to be sorted regardless of which approach to the backplate/flywheel is adopted.  Here’s what to do:

  1. Part of the 4″ is gained by fitting the gaiter the other way round, so that the bellows is offset to the back rather than the front. However to achieve the rest, minor panel work is needed.
  2. Inside the car, remove the console and transmission tunnel mats. Remove the steel cover on the top of the tunnel and extend it by 2.5″ by cutting it across in the middle (at the narrow flat section) and welding in a section of steel.
  3. Position the cover on the tunnel and tap down the flange around the base of the gear lever turret so it matches the tunnel.
  4. Use the extended cover to mark out how far to cut back the top of the tunnel, and drill the tunnel to match the fastening screw holes in the extended part of the cover. After extending the hole in the top of the tunnel, weld captive nuts (1/4″ UNF) underneath the new fastening holes in the tunnel.

Adapting the new gearbox to the existing backplate & flywheel

  1. If using the original starter, cut a hole in the bell-housing’s starter recess to allow the inertia drive of the starter to protrude through it (in much the same way as it did with the original gearbox).  Note:  As starter pinions for the original starter are now becoming hard to come by, you might at this stage consider fitting one of the high torque units now available.  These are a pre-engaging type so pinion wear is virtually eliminated, and also won’t require the hole to be cut.
  2. Fit the gearbox to the engine, securing it with the top right hand bolt (by the outlet to the oil cooler hose) and the bottom bolt directly opposite. (These two have tighter holes than all the others and double as locating dowels)  Note:  The bolts for fitting this bell-housing are longer than those of the 3 synchro. Top and left hand side are 2-7/8″, lower two are 3-1/4″.  (The bottom two longer to allow for the spacers and exhaust bracket). Try to fit 5/16″ UNF bolts here, rather than metric to eliminate the possibilty of mixing different threads and ruining the threads.
  3. Ensure that the hole made in (2) is wide enough to allow the pinion drive assembly to be withdrawn past the ring gear with the gearbox fitted. I found this one the hard way with the first one I did (my car, fortunately) when I went to remove the starter and found that I’d made the hole too neat and had to remove the whole engine just to get the starter out!
  4. You will note that the new bell-housing’s starter bulge is approx 10mm larger than the corresponding part of the backplate. This can be left like that, but the lug which protrudes at the top (where the pre engage starter would have been fitted) will need trimming off to allow clearance when fitted to the body. Mark this out now, tapering in to the line of the backplate at the upper end.
  5. On the bellhousing face, mark the position through the backplate of the top starter bolt and the bolt by the clutch slave cylinder. Remove the gearbox from the engine.
  6. The bottom starter bolt needs to be secured directly to the backplate, rather than through it to the bellhousing as in the 3 synchro installation: Tap the bottom hole in the backplate to 7/16 UNF (It will not require drilling out any further to achieve this). It may be necessary to remove the flywheel to gain access. Bore out the bottom hole in the starter itself to 7/16″ plus clearance. Fit the starter to the engine with this lower bolt. Ensure that the bolt fitted does not protrude past the rear of the backplate and foul the flywheel!
  7. Drill and tap the belhousing 3/8″UNC for the top starter bolt at the position marked in (6) above.
  8. You will have noticed that the other bolt position marked out in (6) – that by the slave cylinder – does not fully land on the bellhousing face. Have a lug welded onto the bellhousing to enable you to drill and tap a 5/16″ UNC hole here.  When dressing up the lug, ensure that there is enough clearance behind it for the slave cylinder to fit. Note: Some people have omitted this particular bolt with apparently no ill effects, but without it, and now that the bottom starter bolt no longer secure the bellhousing as well, there is left a large unsupported section in the engine/gearbox join between the bottom bolt and the top starter bolt.
  9. Trim off the starter lug along the line marked out in (5) above.
  10. The gearbox is now ready to match up to the engine.

The gearbox should now be all ready to fit, except for the propshaft if you are changing from a 3 synchro overdrive gearbox.


As noted above, if you are changing from a 3 synchro overdrive gearbox to a 4 synchro (either non-overdriver or overdrive) you will need to change or shorten your propshaft.  The propshaft situation for B’s is slightly confusing as there were 3 different lengths and 4 different gearbox/diff combinations!  So here goes:

  • If you have a banjo axle, the propshaft to use with a 4 synchro gearbox is AHH7488, 30″ in length.  This is the one fitted to the banjo axle/3 synchro non-overdrive car.
  • If you have a tube axle, the propshaft to use with a 4 synchro gearbox is AHH7487, 31″ in length.  This is the one fitted to the tube axle/4 synchro car and also the banjo axle/3 synchro overdrive car.

Both these propshafts are available new, while AHH7487 is also reasonably easy to get second hand.

Clarifiaction on axle types:

Banjo axle:  Fitted to radsters up to 1966 and some 1967 models.  Identified by the diff pulls out of the front of the axle housing.

Tube axle:  Fitted to all GT’s, all roadsters from 1968 onwards, and some 1967 roadsters.  Identified by a bolt-on cover over the diff housing in the centre of the axle.

Important:  Speedo cable

To avoid kinking the cable  and resultant shortened life and wavery speedo readings:

  • Take care after fitting the speedo cable (a longer cable, GSD115, is needed) to the gearbox that it leaves the gearbox with as gentle a curve as possible as the smaller tunnel makes it a tighter fit.
  • Also take care never to lower the gearbox onto the fixed cross-member with the cable attached.
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